In order to suppress light reflection at the light incident and output interface of an optical device, it has been conventionally known to form an antireflection film by laminating single-layer or multiple-layer optical films each having a different refractive index so as for the antireflection film to have a thickness of several ten to several hundred nanometers. In order to form such antireflection film, there is used a vacuum film-forming method such as vapor deposition or sputtering or a wet film-forming method such as dip coating or spin coating.
It is known to use, as a material used for forming the outermost layer of the antireflection film, a transparent material having a low refractive index selected from inorganic materials such as silica, magnesium fluoride, and calcium fluoride and organic materials such as a silicone resin and an amorphous fluororesin.
In recent years, it has been known to use a low refractive index film utilizing the refractive index of air 1.0 for the antireflection film in order to accomplish a lower reflectance. It is possible to reduce the refractive index of a layer of silica or magnesium fluoride by forming voids in the layer. For example, the refractive index of a thin film of magnesium fluoride, i.e., 1.38 can be reduced to 1.27 by forming voids accounting for 30% of the volume of the thin film therein.
It is known that an antireflection film having a low refractive index is provided by using a method of forming voids involving forming fine particles of silica or magnesium fluoride into a film with a binder and forming voids in the binder between the fine particles (Patent Literatures 1 and 2).
Moreover, there is known, as another method of forming voids, a method involving forming voids in particles by using hollow silica particles. This is a method of forming an antireflection film by using the hollow particles (Patent Literature 3).
On the other hand, it is known that, when hollow silica particles are used for making an optical member such as an antireflection film, problems may occur in the transparency and outer appearance of the optical member. This is because the hollow silica particles have poor affinity to an organic solvent and an organic polymer which are mixed with the particles, and hence the hollow silica particles aggregate when the mixture is formed into a paint, and light scattering is caused in the optical member. In order to solve the problem, there is known a method involving forming a film by using a paint free of an organic polymer, thereby suppressing the aggregation to produce a low refractive index film not causing light scattering, and yielding an antireflection film using the low refractive index film (Patent Literature 4).